1. Field of the Invention
The invention relates to a conical roller bearing device and, more particularly, to a conical roller bearing device that is suitable for supporting a rotary shaft, such as a camshaft and an output shaft, in an engine or transmission for a vehicle.
2. Description of Related Art
Generally, a conical roller bearing withstands a large load. The conical roller bearing is able to withstand a radial load, an axial load and a combined load of the radial load and the axial load, and also has a high bearing stiffness, so the conical roller bearing is not only used in automobiles but also widely used in other fields. In recent years, particularly, centered on automobiles, in response to a request for resource savings and low environmental loads, there has been an effort to reduce friction torque of a bearing device of a rotary shaft of an engine or transmission of an automobile in order to reduce a friction loss to thereby improve fuel economy. For this purpose, recently, not only a plain bearing but also a rolling bearing, including a conical roller bearing, has been widely used.
Incidentally, such a conical roller bearing is generally used in a state where a preload is applied in the axial direction of the conical roller bearing in order to ensure bearing stiffness and to suppress generation of noise (unusual noise) and vibrations. A known method of applying a preload is fixed position preloading and constant pressure preloading. The fixed position preloading is to keep the relative position in the axial direction between facing inner and outer rings constant during usage of a conical roller bearing, while the constant pressure preloading is to apply an appropriate preload to inner and outer rings using a coil spring, a belleville spring, or the like, to thereby keep a preload amount substantially constant even when the relative position changes during usage of a conical roller bearing.
Incidentally, in the above described fixed position preloading, it is known that a so-called loss of preload due to a difference in coefficient of thermal expansion between both members that are fitted to the conical roller bearing. Then, in order to solve the above problem, Japanese Utility Model Application Publication No. 05-006250 (JP-U-05-006250) describes a technique for eliminating an insufficient preload in such a manner that a thrust washer made of a shape-memory alloy is interposed between a bearing housing and an end portion of an outer ring and then the thrust washer is elastically deformed with an increase in temperature to urge the outer ring in the axial direction.
In addition, Japanese Patent Application Publication No. 2008-240915 (JP-A-2008-240915) describes a rolling bearing device. In the rolling bearing device, a bearing housing that is one of both members to which the conical roller bearing is fitted has a first linear expansion coefficient, a rotary shaft that is the other one of both members has a second linear expansion coefficient smaller than the first linear expansion coefficient, and an internal screw portion is formed on part of the inner periphery of the bearing housing. The rolling bearing device includes a pressing member, a sensor and preload maintaining means. The pressing member has an external screw portion screwed to the internal screw portion on its outer periphery, and is displaceable in the axial direction. The sensor detects a difference in thermal expansion between the bearing housing and the rotary shaft. The preload maintaining means allows the pressing member to rotate on the basis of the result detected by the sensor to press the outer ring in the axial direction to thereby maintain the preload on the rolling bearing in the axial direction.
Incidentally, in each of the conical roller bearing device described in JP-U-05-006250 and the conical roller bearing device described in JP-A-2008-240915, the outer ring or the inner ring, which is one of the outer ring and the inner ring that constitute the conical roller bearing, is pressed in the axial direction to be moved in the axial direction relative to the inner ring or the outer ring, which is the other one of the outer ring and the inner ring, to thereby maintain a preload. However, when the outer ring or the inner ring is pressed in the axial direction to displace the outer ring or the inner ring in the axial direction, the outer ring is not rotating relative to the bearing housing or the inner ring is not rotating relative to the rotary shaft, so the friction resistance between the outer ring and the bearing housing or the friction resistance between the inner ring and the rotary shaft is large. As a result, in order to relatively displace the outer ring or the inner ring in the axial direction, large driving force that overcomes the friction resistance is required, so there has been a problem that it is necessary to provide a complex and expensive mechanism, such as a booster mechanism. In addition, when the outer ring or the inner ring is relatively displaced in the axial direction, friction arises on the sliding surface. As a result, there is a possibility that it becomes difficult to displace the outer ring or the inner ring over long-term usage.